Perovskite-Inspired Photovoltaic Materials: Toward Best Practices in Materials Characterization and Calculations

Recently,
there has been an explosive growth in research based on hybrid lead–halide
perovskites for photovoltaics owing to rapid improvements in efficiency.
The advent of these materials for solar applications has led to widespread
interest in understanding the key enabling properties of these materials.
This has resulted in renewed interest in related compounds and a search
for materials that may replicate the defect-tolerant properties and
long lifetimes of the hybrid lead-halide perovskites. Given the rapid
pace of development of the field, the rises in efficiencies of these
systems have outpaced the more basic understanding of these materials.
Measuring or calculating the basic properties, such as crystal/electronic
structure and composition, can be challenging because some of these
materials have anisotropic structures, and/or are composed of both
heavy metal cations and volatile, mobile, light elements. Some consequences
are beam damage during characterization, composition change under
vacuum, or compound effects, such as the alteration of the electronic
structure through the influence of the substrate. These effects make
it challenging to understand the basic properties integral to optoelectronic
operation. Compounding these difficulties is the rapid pace with which
the field progresses. This has created an ongoing need to continually
evaluate best practices with respect to characterization and calculations,
as well as to identify inconsistencies in reported values to determine
if those inconsistencies are rooted in characterization methodology
or materials synthesis. This article describes the difficulties in
characterizing hybrid lead–halide perovskites and new materials
and how these challenges may be overcome. The topic was discussed
at a seminar at the 2015 Materials Research Society Fall Meeting & Exhibit. This article highlights the lessons learned from the seminar and the insights of some of the attendees, with reference to both recent literature and controlled experiments to illustrate the challenges discussed. The focus in this article is on crystallography, composition measurements, photoemission spectroscopy, and calculations on perovskites and new, related absorbers. We suggest how the reporting of the important artifacts could be streamlined between groups to ensure reproducibility as the field progresses.